Referring to FIG. 1A, one example of a system for drilling an earth formation is shown. The drilling system includes a drilling rig 10 used to turn a drilling tool assembly 12 that extends into a well bore 14. The drilling tool assembly 12 includes a drill string 16, and a bottomhole assembly (BHA) 18, which is attached to the distal end of the drill string 16. The “distal end” of the drill string is the end furthest from the drilling rig 10.
The drill string 16 includes several joints of drill pipe 16a connected end-to-end through tool joints 16b. The drill string 16 is used to transmit drilling fluid (through its hollow core) and to transmit rotational power from the drilling rig 10 to the BHA 18. In some cases the drill string 16 further includes additional components such as subs, pup joints, etc.
The BHA 18 includes a drill bit 20. A BHA may also include additional components attached between the drill string 16 and the drill bit 20. Examples of additional BHA components include drill collars, stabilizers, measurement-while-drilling (MWD) tools, logging-while-drilling (LWD) tools, subs, hole enlargement devices (e.g., hole openers and reamers), jars, thrusters, downhole motors, and rotary steerable systems.
In the drilling of oil and gas wells, concentric casing strings are installed and cemented in the well bore as drilling progresses to increasing depths. Each new casing string may run from the surface or may include a liner suspended from a previously installed casing string. The new casing string may be within the previously installed casing string, thereby limiting the annular area available for the cementing operation. Further, as successively smaller diameter casing strings are used, the flow area for the production of oil and gas is reduced. To increase the annular space for the cementing operation, and to increase the production flow area, it may be desirable to enlarge the well bore below the terminal end of the previously cased portion of the well bore. By enlarging the well bore, a larger annular area is provided for subsequently installing and cementing a larger casing string than would have been possible otherwise. Accordingly, by enlarging the well bore below the previously cased portion of the well bore, comparatively larger diameter casing may be used at increased depths, thereby providing more flow area for the production of oil and gas.
Various methods have been devised for passing a drilling assembly through an existing cased portion of a well bore and enlarging the well bore below the casing. One such method is the use of an underreamer, which has basically two operative states—a closed, retracted, or collapsed state, where the diameter of the tool is sufficiently small to allow the tool to pass through the existing cased portion of the well bore, and an open or expanded state, where arms with cutters on the ends thereof extend from the body of the tool. In this latter position, the underreamer enlarges the well bore diameter as the tool is rotated and lowered in the well bore.